The present invention relates to a stapler and, more particularly, a stapler that is resistant to jamming.
An electric stapler that automatically drives and clinches a staple is particularly useful when frequent stapling is required or when stapling thicker, penetration resistant workpieces comprising many layers of material or layers of heavier material. Both battery and AC powered electric staplers are commercially available. Battery power offers cordless portability, while AC power reduces the weight of the stapler and avoids the need to periodically replace or recharge the battery.
In a typical electric stapler, a workpiece, typically comprising a plurality of layers of paper, leather, fabric or similar material, is compressed between an anvil and a portion of a movable magazine in which a staple assembly comprising a plurality of staples detachably bonded together is retained. The staples are typically fabricated from wire that is bent to form a crown with a leg projecting normal to the crown at each end. An electric motor urges a driver, slidable inside the magazine, toward the front surface of the workpiece. A surface of the driver engages the crown of the first staple in the staple assembly, separating the staple from the staple assembly and forcing the staple out of the magazine causing the ends of the legs to penetrate the front surface of the workpiece. The legs of the staple are preferably longer than the thickness of the workpiece so that portions of the legs that project from the back surface of the workpiece can be clinched, bent back toward the workpiece, to bind the layers of the workpiece together and secure the staple in the workpiece.
Staples are commonly clinched by forcing the ends of the legs against a surface of a fixed anvil until the columnar legs buckle. The buckling may be facilitated by curved surfaces in the anvil that are arranged to receive and deflect the ends of the legs as they are pushed through the back surface of the workpiece. Continued movement of the driver, to force the crown of the staple into contact with the front surface of the workpiece, bends the legs back into contact with the back surface of the workpiece completing the clinching. However, clinching a staple by forcing the ends of the legs against the surface of a fixed anvil often proves problematic when stapling thicker workpieces or workpieces comprising layers of heavier or penetration resistant material. Thicker, penetration resistant workpieces commonly require a heavy-duty staple with stronger legs that will not bend when forced to pierce the workpiece. Clinching a heavy-duty staple by forcing the ends of the legs against the surface of a fixed anvil is problematic because the stronger legs are more resistant to bucking and a relatively short portion of the leg may protrude from the back surface of a thicker workpiece. The higher force that must be exerted by the driver to clinch a stronger staple often leads to inadequate clinching or lateral displacement of the staple causing a portion of the staple to lodge between the moving parts of the stapler jamming the mechanism.
Staplers using heavy-duty staples or used to staple heavier, thicker workpieces often include a movable clincher that operates in conjunction with the driver to bend the legs of the staple. Ura, U.S. Pat. No. 6,820,790 B2 discloses two movable clinching mechanisms suitable for use with an electric stapler. The clinching mechanisms comprise a plurality of spaced apart, parallel, stationary anvil plates arranged to support a workpiece on their upper edges. A clincher, comprising a clinching arm movable between a retracted position and a clinching position where a surface of the arm is substantially even with the workpiece supporting edges of the plates, is rotatably supported between each pair of stationary anvil plates. A recess, formed between the parallel plates when the clincher arm is retracted, is arranged to receive the legs of a staple as the legs erupt from the back surface of the workpiece. When the driver has pushed the staple into the workpiece, a clincher drive mechanism rotates the clinchers causing the arms of the clinchers to sweep through their respective recesses and engage and bend the protruding legs of the staple. While movable clinching mechanisms promote more consistent clinching of heavier staples, the clinching mechanisms comprise several small, flexible parts that are separated by clearances and portions of the staple can lodge in the clearances preventing further operation of the stapler. Moreover, the parts of the clincher mechanisms are small and wear rapidly, exacerbating jamming and necessitating frequent replacement of the mechanism.
Staples can also become lodged in the clearance between the movable driver and the inner surface of the nosepiece which guides the staple and the driver as the staple is pushed out of the magazine and into the workpiece. The driver moves normal to the longitudinal axis of a staple assembly which is retained in the magazine and urged toward the inner surface of the nosepiece at the front of the magazine. As the driver moves toward the anvil, guided by the inner surface of the nosepiece, a surface of the driver contacts the crown of the first staple in the staple assembly, separating the staple from the assembly and forcing the ends of the legs to pierce the front surface of the workpiece. The higher forces required to pierce penetration resistant workpieces and clinch heavy-duty staples increase the likelihood that the crown of the staple will be displaced laterally on the contact surface of the driver and become lodged between the driver and the inner surface of the nosepiece preventing further operation of the stapler, a problem that is exacerbated by wear of the inner surface of the nosepiece.
What is desired, therefore, is a stapler including a staple clincher that resists jamming.